debug/dwarf: optimize buf.string
debug/dwarf: optimize buf.string

(*buf).string previously manually searched
through its underlying byte slice until we
encountered a '0'. This change instead uses
bytes.IndexByte that results in a speed up:

$ benchstat before.txt after.txt

name         old time/op    new time/op    delta
BufString-8     257ns ± 1%     174ns ± 1%  -32.37%  (p=0.000 n=9+8)

name         old speed      new speed      delta
BufString-8   495MB/s ± 1%   732MB/s ± 1%  +47.76%  (p=0.000 n=10+8)

name         old alloc/op   new alloc/op   delta
BufString-8      162B ± 0%      162B ± 0%     ~     (all equal)

name         old allocs/op  new allocs/op  delta
BufString-8      3.00 ± 0%      3.00 ± 0%     ~     (all equal)

Change-Id: I7cf241742cc091d5d30d987a168b02d83955b1cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/196657
Run-TryBot: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>

CAFxX/go,akutz/go,cloudflare/go,cloudflare/go,golang/go,golang/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,akutz/go,nawawi/go,akutz/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,akutz/go,akutz/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,akutz/go,akutz/go,nawawi/go,golang/go,akutz/go,cloudflare/go,golang/go,golang/go